Grey dunes are fixed, stable

sand dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...

that are covered by a continuous layer of herbaceous vegetation. These dunes are typically located 50–100 meters from the ocean shore and are found on the landward side of

foredunes A foredune is a dune ridge that runs parallel to the shore of an ocean, lake, bay, or estuary. Foredunes consist of sand deposited by wind on a vegetated part of the shore. Foredunes can be classified generally as ''incipient'' or ''established'' ...

(also known as yellow dunes). Grey dunes are named for their characteristic grey color which is a result of the ground cover of

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.humus In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...

Formation

Grey dunes are formed when pioneer species inhabit embryo dunes and begin a successional process toward dune stabilization and soil formation. Pioneer plants capture saltating sand grains carried by the wind and deposit them to continue build up the dune. Grey dunes are then formed after the root structures of pioneering plants stabilize

. Sand stabilization allows for dead plant material to be decomposed within the soil, adding nutrients and water-retention abilities to the soil. As soil starts to slowly form, new plant species colonize the stable dunes ultimately developing more extensive root structures and plant diversity. When the dune has become continuously vegetated,

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...

grows in the understory, providing a water-repellent surface that reduces erosion (through rainfall and wind), this results in the official formation of the grey dune. Pluvial processes dominate the geomorphology of grey dunes, with rain splash and slope wash being the most common processes taking place. Erosion can take place in the form of rain splash and slope wash. Rain splash occurs when a raindrop impacts the bare soil surface and disperses soil particles to the surrounding area. However, as grey dunes begin to develop, lichens and mosses protect the dune surfaces and dissipate raindrop impacts. Slope wash (overland flow) is particularly evident in sparsely vegetated dune slopes. Slope wash occurs when rainfall infiltration is impeded, which can result in

rills In hillslope geomorphology, a rill is a shallow channel (no more than a few inches/decimeters deep) cut into soil by the erosive action of flowing surface water. Similar but smaller incised channels are known as microrills; larger incised ch ...

and alluvial fans.

Dune pH

Depending on the sand composition, a grey dune will become either acidic or calcareous. An acidic soil

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

will form if the sand is composed completely of quartz grains. Older grey dunes can also exhibit acidic conditions if there has been enough rainfall to leach out the soil nutrients. Calcareous soil substrates form when seashells decompose to create a lime-rich environment. The vegetation on grey dunes is dependent on the pH of the soil substrate.

Vegetation

Many rare and endangered plant species can be found within grey dune ecosystems, providing niche habitats for animals and insects. A wide variety of orchids, grasses, and lichens are found in grey dune plant communities but species will depend on locations throughout the European continent.

Location

Grey dunes are found along much of the European coast and along large, inland bodies of water. Broadly speaking, they can be found along the

Celtic Celtic, Celtics or Keltic may refer to: Language and ethnicity *pertaining to Celts, a collection of Indo-European peoples in Europe and Anatolia **Celts (modern) *Celtic languages **Proto-Celtic language * Celtic music *Celtic nations Sports Fo ...

, Baltic, and North Seas, the Eastern and Mid Atlantic coast, the Western and Eastern Mediterranean coasts, and the Black sea.

Loss of Grey Dunes

Grey dunes are highly susceptible to change from both natural and human-induced events. Since the 1960s, grey dune ecosystems have declined from the encroachment of grass and

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...

species, and more recently, by the invasion by the alien invasive Heath Star Moss (

Campylopus introflexus ''Campylopus introflexus'', also known as the heath star moss, is a species of moss. The first description of the species was made by Johannes Hedwig as ''Dicranum introflexum'' in 1801. Description Individual plants measure , with lanceolate l ...

). Causes of the encroachment have been related to large-scale sand dune stabilization, high nitrogen deposition from the vegetation, and a decline in

rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit speci ...

populations within the dunes.{{Cite journal, title = Rapid Improvement of Grey Dunes After Shallow Sod Cutting, last = van Til, first = Mark, date = 2007, journal = Coastline Reports 7, first2 = Annemieke, last2 = Kooijman Grass and

shrub encroachment Woody plant encroachment (also called bush encroachment, shrub encroachment, woody encroachment, bush thickening, or woody plant proliferation) is a natural phenomenon characterised by the increase in density of woody plants, bushes and shrubs, ...

has resulted in the

fragmentation Fragmentation or fragmented may refer to: Computers * Fragmentation (computing), a phenomenon of computer storage * File system fragmentation, the tendency of a file system to lay out the contents of files non-continuously * Fragmented distributi ...

of grey dune landscapes (and overall reduction in habitat), a loss of biodiversity, and a decline of endangered and rare species that are endemic to grey dunes.

References